This invention relates generally to vehicle rearview mirror systems and, more particularly, to electro-optic mirror assemblies, such as electrochromic rearview mirror assemblies for a vehicle.
Electrochromic rearview mirror assemblies include an electrochromic reflective element made up of a reflecting surface and an electrochromic cell positioned between the driver and the reflecting surface. The electrochromic cell responds to a direct current (DC) voltage applied across a pair of terminals by varying the light transmittance through the cell. In this manner, the reflectance level of the reflective element can be varied by varying the DC voltage applied to the electrochromic cell. The electrochromic cell has characteristics which make control of the reflectance level of the reflective element difficult. The electrochromic cell operates at a relatively low voltage, typically which may not exceed approximately 3 volts DC, more typically not more than about 1.5 volts DC, for more than a brief period of time or else useful life of the reflective element is compromised. Furthermore, the amount of drive current necessary to color or bleach the cell varies both with the temperature of the cell and the amount of change in light transmittance undertaken. Therefore, optimum control of the electrochromic cell requires more than merely applying a DC voltage corresponding to the desired reflectance level.
One approach to controlling the reflectance level of an electrochromic cell is disclosed in commonly assigned U.S. Pat. No. 5,715,093 issued to the present inventor and Niall R. Lynam, entitled AUTOMATIC REARVIEW MIRROR SYSTEM WITH AUTOMATIC HEADLIGHT ACTIVATION. In this co-pending application, the electrochromic cell is driven by an analog feedback system which translates a desired reflectance level, produced by an analog circuit, to a signal applied to the electrochromic cell which drives the cell to the desired reflectance level. While such drive system is effective, it requires the use of analog components. Such analog components would be redundant in a digital electrochromic mirror system and, therefore, would unnecessarily add to the cost of the system. However, substitution of digital components for the previously used analog components is not a straightforward matter. Digital components typically operate between discrete output states which may include binary devices, such as transistors, switches, and the like, which exhibit a low and a high state, and tristate devices, such as types of microprocessors which exhibit a neutral, a low, and a high state. Such components are useful in processing data but are not readily adapted to controlling the reflectance level of an electrochromic rearview mirror. In particular, a typical electrochromic mirror utilized as an interior mirror of a vehicle may have a surface area in the range of 90 cm2 to 150 cm2 and typically in the range of 110 cm2 to 130 cm2. A steady state current draw, after color transitions have settled, is typically in the range of between approximately 60 milliamperes and 180 milliamperes with a range of 80 milliamperes to 150 milliamperes being typical. Exterior rearview mirrors can be even larger with a surface area of approximately 350 cm2, and greater, and a commensurate increase in current density.
The present invention provides a digital electrochromic mirror system which utilizes primarily digital components to drive an electrochromic cell of an electrochromic mirror system to a desired reflectance level which not only meets, but desirably exceeds the performance of prior analog systems.
According to an aspect of the invention, an electrochromic rearview mirror system for a vehicle includes an electrochromic reflective element having an electrochromic cell wherein the reflective element colors to a partial reflectance level in response to a drive signal applied to the electrochromic cell. The rearview mirror assembly additionally includes a drive circuit which applies a pulsed drive signal to the electrochromic cell in order to establish the partial reflectance level of the reflective element. The drive circuit controls the partial reflectance level at least as a function of the duty cycle of the pulsed drive signal.
According to another aspect of the invention, an electrochromic rearview mirror assembly for a vehicle includes such an electrochromic reflective element and a drive circuit which applies a drive signal to the electrochromic cell in order to establish the partial reflectance level of the reflective element. The drive circuit includes a digital controller, a binary switching device responsive to an output of the controller for applying a source to the electrochromic cell, and an input of the controller. The input of the controller is preferably responsive to the voltage developed across the electrochromic cell by the source. The digital controller closes and opens the binary switching device according to a particular duty cycle in order to control the partial reflectance level at least as a function of the duty cycle. The digital controller additionally adjusts the source as a function of the voltage developed across the electrochromic cell.
According to yet an additional aspect of the invention, an electrochromic rearview mirror assembly for a vehicle includes such an electrochromic reflective element and drive circuit which applies a drive signal to the electrochromic cell in order to establish the partial reflectance level of the reflective element. The drive circuit includes a digital controller, a first binary switching device responsive to an output of the controller for applying a source to the electrochromic cell, and a second binary switching device which is responsive to an output of the controller for draining charge from the electrochromic cell. The controller alternatingly closes the switching devices according to a particular duty cycle in order to control the partial reflectance level as a function of the duty cycle. The digital controller closes and opens the binary switching device at a repetition rate of at least approximately 10 cycles per second, more preferably at least approximately 20 cycles per second, and most preferably at least approximately 25 cycles per second.
An electrochromic rearview mirror assembly, according to the various aspects of the invention, may additionally include other functions of the rearview mirror including a display which displays the vehicle heading, determined by a compass, the outdoor temperature, determined by an outdoor temperature sensor, or both the vehicle heading and outdoor temperature. The digital controller, which is preferably a microcomputer, may additionally control the intensity of the display. The intensity of the display may be controlled as a function of light levels around the vehicle. Additionally, in particular embodiments, the display may be positioned behind the electrochromic cell wherein the display is viewed through the electrochromic cell. In such embodiments, the microcomputer may additionally adjust the intensity of the display as a function of the reflectance level of the reflective element. In this manner, the display, as perceived by the driver, does not vary in intensity as the reflectance level of the reflective element changes. However, the intensity of the display may be adjusted to accommodate the physiological response of the driver""s eyes.
These and other objects, advantages and features of this invention will become apparent upon review of the following specification in conjunction with the drawings.